The mixing behavior of 1-(perfluorobutyl)undecanoic acid-pentadecanoic acid (C15), 1-(perfluorohexyl)undecanoic acid-heptadecanoic acid (C17), and 1-(perfluorooctyl) undecanoic acid-nonadecanoic acid (C19) mixtures was investigated at the air-water interface. The compression isotherms of the fluorocarbon acid-hydrocarbon acid mixtures were recorded at various compositions on hydrochloric acid (pH 1.9, 37 +/-2degreesC) as a subphase. The phase transition, limiting molecular area, area at collapse pressure, and collapse pressure were determined for all pi-A isotherms. The mixing behavior was assessed by analyzing the concentration dependence of the average molecular area at constant film pressure (area/mole fraction or A-X diagram) and the concentration dependence of the phase transition, where possible. All three acid mixtures show a negative deviation from ideal behavior at surface pressures between 5 and 20 mN/m, which is indicative of an attractive interaction of both compounds in the mixed monolayer at the air-water interface. The miscibility apparently decreases with increasing chain length of the carboxylic acids (C15 > C17 > C19).